Source of beams for producing a high intensity charge carrier beam



Jan. 1, 1 963 F. SCHLEICH SOURCE OF BEAMS FOR PRODUCING A HIGH INTENSITY CHARGE CARRIER BEAM Filed Aug. 26, 1959 n m. 1 M w I n w a a Unite tates i tet 3,071,707 SOURCE OF BEAMS FUR PRUDUQING A HIGH INTENSHTY CHARGE GARRER BEAW Fritz Sehleich, Wurttemberg, Germany, assigner to Carl Zeiss, Heidenheim (Brena), Wurttemherg, Germany Filed Aug. 26, 1959, Ser. No. 836,172 Claims priority, application Germany Sept. 6, 1958 4 Claims. (til. 3i384) The present invention relates to a source for producing a high intensity beam of charged particles of circular cross section.

In thenormally used beam generating systems with a cathode comprising a tungsten wire bent into hairpin-like form, astigmatic error in beam generation occurs in the emission system components, the cathode, control electrode and anode. The astigmatic error can be attributed to two basic causes. The first cause of the astigmatism is due to the different curvature of the wire cathode in the direction of the wire and in the direction transversely of the wire. The second source of error results from the extension of the emitting surface of the cathode in two directions along the emitting wire. Thus, focussing of the beam by the normal symmetrical lens, as, for example, an electrostatic field lens for an electron beam, results in a beam, the intensity of which is distributed over an elliptical area at the focal plane.

However, in many applications, it is desirable to focus the beam upon the focal plane with a circular cross section. For example, in the working of materials by such beams of charged particles, a circular cross section is particularly desirable.

Production of a beam of circular cross section has been accomplished by the art by inserting a circular diaphragm in the path of the beam. Although this is efiec tive in restricting the cross section of the beam to a circular cross section, it has the disadvantage that the intensity of the beam is decreased.

lt has been known to provide a cylindrical lens in an electron focussing source in which the cylindrical lens is placed in or beyond the electromagnetic lens serving to focus the beam. The cylindrical lens, variable both as to azimuth and field strength, serves to correct the astigmatism of the lens but does not correct the asigmatism of the beam generating system. Thus, the beam in such known arrangements must be circularly limited by means of a diaphragm prior to entering the lens. This results in a loss of intensity in the focussed beam.

It is, therefore, a primary object of this invention to correct the astigmatism in the source of a beam of charged particles to enable focussing of said beam to a circular cross-sectional shape at the focal plane.

It is a further object of this invention to provide improved methods and means for correcting beam astigmatism in simple, economic fashion.

Other objects and advantages will be pointed out hereinafter.

In accordance with these objects there is provided a source ofa beam of charged particles to produce a high intensity beam of circular cross section without an aperture stop.

The source comprises a wire cathode heated to serve as an emission source for the particles, a control electrode, an anode and a cylindrical magnetic lens adjustable both as to field strength and as to azimuth position of the main axis thereof. The anode and cylindrical lens are combined into a single structural unit. The construction of such a source of a high intensity beam of charged particles is thus simple and economical.

In one embodiment of this invention the cylindrical lens for correcting the source of astigmatism comprises two permanent magnets radially arranged on a rotatable disc connected to the anode. The magnets are jointly rotatable and reciprocably displaceable in a radial direction to provide means for respectively adjusting the azimuth of the main axis and the field strength of the lens.

in another embodiment of this invention the cylindrical lens comprises four permanent magnets in the same plane. Two magnets are coupled to a first disc and are arranged radially with separation while the other two magnets are arranged in the same manner on a second disc. The discs are rotatable with respect to each other and jointly with respect to the anode to respectively alter the field strength and azimuthal alignment of the major axis of the lens.

If the anode of the source is combined with a single cylindrical lens, the astigmatism can be corrected only with respect to one cause of astigmatic error. In order to compensate for both causes of astigmatism error, it is often desirable to couple a second cylindrical lens beyond the first cylindrical lens as seen in the direction of beam travel.

Alternately, it may be possible to shape the cathode in order to eliminate the one cause of astigmatism error. For example, the cathode may be fabricated of wire having a semi-circular profile so that an approximately spherical point is formed at the front end of the hairpin-like bent wire cathode. In such case, it has been found that a single cylindrical lens will correct the astigmatism, namely, that part of the astigmatic error caused by extension of the emitting surface along the wire beyond the spherical point.

It has also been found advisable to provide a mounting guide for fixing the azimuth of the cathode. These guides will ensure location of the cathode with the same azimuthal relationship when the cathode is changed. With such guides it is possible to fixedly adjust the azimuth of the correcting lens and the refractive power of the lens for a certain emitting range. In such case, it is advantageous to fabricate the cylindrical lens from permanent magnets.

In those cases where it is necessary to change the azimuth and the refractive power of the cylindrical lens to vary the compensation required for beam focussing, it is often advisable to use a cylindrical lens having multiple electromagnetic poles, such as an eight pole lens, each pole of which is energized by an associated coil. The power supply is developed so that continuous transition of each pole from North to South may be efiected by simple regulating apparatus. Thus, a selectable azimuth and variable refractive power of the resulting field may be selected for the application intended. Variations in astigmatic error during operation can be corrected by adjustment of the field from a position outside of the working beam source.

The invention will now be described by way of preferred embodiments shown in the accompanying drawings of which:

FIGURE 1 is a cross section of one embodiment of a beam generating system in accordance with this invention;

FIGURE 2 is a section taken along lines IIII of FIGURE 1;

FIGURE 3 is a cross-section of another embodiment of this invention;

FIGURE 4 is a section taken along lines IV-IV of FIGURE 3;

FIGURE 5 is a cross-sectioned view of another embodiment of this invention; and

FIGURE 6 is a section taken along lines VIVI of FIGURE 5.

The source for a beam of charged particles shown in FIGURES 1 and 2 comprise a cathode 1, a control electrode 2 and an anode 3. The cathode is formed of suitable material which may be heated by the passage of current therethrough to effect emission therefrom of charged particles, such as electrons. The control electrode 2 is positioned with respect to the cathode to control the beam generation by the bias thereon. The anode 3 is located with respect to the cathode and the control electrode and is biased to provide a suitable accelerating field for the charged particles. The beam formed thereby passes through an aperture in the anode in a conventional fashion and may be focussed by suitable focussing elements onto a focal plane.

A disc 4 is rotatably coupled to the anode by threaded engagement therewith. The disc carries two permanent magnets 5 and 6 positioned on mounting members and 11 respectively which slideably engage radially extending slots in the disc 4. Like pole of the magnets 5 and 6 face each other to generate a magnetic field of the pattern required for a cylindrical lens. The magnets 5 and 6 are biased outwardly by respective springs 7 and 8. An annular ring 9 surrounds the mounting members 10 and 11 in engagement with the outer beveled surfaces thereof in such manner that upward movement of the ring will force the magnets 5 and 6 toward each other. Thus, the refractive power of the cylindrical lens formed by the magnetic field of the magnets 5 and 6 may be adjusted by vertical adjustment of the annular ring. Rotation of the disc 4 will rotate the magnets to vary the azimuthal positioning of the cylindrical lens axis.

In the beam generating source shown in FIGURES 3 and 4, there is shown the cathode 1, control electrode 2, and anode 3 similar to those set forth in connection with FIGURE 1. A disc 12, to the wall of which are affixed permanent magnets 13 and 14 is rotatably coupled to the anode by threaded engagement therewith. A second disc 15 to the top surface of which is aflixed permanent magnets 16 and 17 is rotatably connected to disc 12 by threaded engagement. It will be noted that to position the magnets in the same plane, the discs will be assembled to each other prior to the bonding of one set of magnets. If discs 12 and 15 are rotated with respect to one another, the refractive power of the cylindrical lens comprising magnet elements 13, 14, 16 and 17 is varied. Rotation of both discs jointly Will vary the azimuthal positioning of the axis of the cylindrical lens formed by the magnetic fields of the permanent magnets.

In those cases in which it is desirable to vary the azimuthal position of the axis of the cylindrical lens and the refractive power of the lens during applications, such as the working of material when the source is enclosed in an evacuated chamber, the embodiment shown in FIGURES 5 and 6 may advantageously be employed.

In FIGURES 5 and 6 there is shown a so-called remote focussing cathode comprising the actual cathode 18,

and a specially constructed control electrode 19. The

anode 20 is similar to that of FIGURES 1-4. A disc 21 is rotatably coupled to the anode in threaded engagement therewith and carries eight electromagnetic coils 22 to 29, each of which is wound on a suitably shaped pole shoe.

The coils and pole shoes of the cylindrical lens shown 5 are potted in a resin 30. The potting seals the coils so that air between the wires of the coils need not be drained out by pumping during evacuation of the system in which the beam generating source is used.

The coils are energized from a suitable supply. The polarity of the voltage applied to each coil may be reversed and the amplitude of the current applied thereto regulated. Thus, by simple regulating elements (known 4 to the art and thus not shown) the refractive power of the lens and the azimuthal positioning of the axis of the lens may be selected even during operation of the electron beam source to compensate for changed astigmatic error.

It will be noted that other forms of electromagnetic cylindrical lenses may be employed.

Thus, there is provided, represented 'by FIGURES 1-4 particularly, simple and useful corrective elements for beam generating sources. These embodiments may advantageously be employed for fixed operating conditions as often occur in practice. The embodiment represented in FIGURES 5 and 6 may advantageously be employed where operating conditions vary and the correction for astigmatism error must be variable.

The beam generating sources in which the astigmatic error has been compensated in accordance with this invention is particularly applicable to applications in which beams of high intensity are required, such as in the different ways of working material, for example, drilling, milling or welding. In many of such applications, it is, of course, advantageous to control the beam by suitable pulse modulation of the bias of the control electrode to generate a pulsed beam.

This invention can be variously embodied and modifled within the scope of the subjoined claims.

What is claimed is:

1. A generating source for generating a high intensity beam of charged particles of circular cross-section comprising a wire cathode, said cathode being formed in hairpin-like shape, a control electrode, an anode, said control electrode being interposed between said anode and said cathode, and a magnetic field cylindrical lens mounted on said anode to correct the astigmatism of said source, said lens being adjustable both with respect to refractive power and with respect to azimuthal positioning of the major axis of said lens.

2. A source in accordance with claim 1 in which said lens comprises two radial magnets, a disc, said disc carrying said magnets in radially extending paths thereon, said disc being rotatably mounted on the anode, and means for radially displacing said permanent magnets.

3. A source in accordance with claim 1 in which said lens comprises a disc, a plurality of electromagnetic coils mounted on said disc and means for selectively energizing said coils to provide an electromagnetic lens of variable refractive power and azimuthal alignment of the axis thereof.

4. A source in accordance with claim 1 which includes means for locating the azimuth position of said cathode.

References Cited in the fiie of this patent UNITED STATES PATENTS 1,981,245 Pearcy Oct. 20, 1934 2,157,182 Maloff May 9, 1939 2,165,803 Maloff July 11, 1939 2,362,908 Liebman Oct. 14, 1944 2,369,782 Hillier Feb. 20, 1945 2,498,354 Bocciarelli Feb. 21, 1950 2,591,159 Kabuss Apr. 1, 1952 2,634,381 Kafka Apr. 7, 1953 2,714,678 Wolff Aug. 2, 1955 2,754,443 Asmus July 10, 1956 2,883,569 Kaiser et a1. Apr. 21, 1959 2,910,603 Van Dorsten et al. Oct. 27, 1959 FOREIGN PATENTS 891,119 Germany Sept. 24, 1953 

1. A GENERATING SOURCE FOR GENERATING A HIGH INTENSITY BEAM OF CHARGED PARTICLES OF CIRCULAR CROSS-SECTION COMPRISING A WIRE CATHODE, SAID CATHODE BEING FORMED IN HAIRPIN-LIKE SHAPE, A CONTROL ELECTRODE, AN ANODE, SAID CONTROL ELECTRODE BEING INTERPOSED BETWEEN SAID ANODE AND SAID CATHODE, AND A MAGNETIC FIELD CYLINDRICAL LENS 